Saw chains of chain saws mounted at harvesters of the single-grip type for the felling and processing of tree trunks often run with large peripheral speeds around the guide bar. The saw arrangement that carries out the sawing is mounted in the protection of a shielded sawing box. A conventional saw arrangement comprises a frame, a saw motor, a driving wheel, a guide bar, a saw chain, a holder for the guide bar and an element to feed the guide bar out of and into the sawing box. The saw motor is arranged to drive the driving wheel and to rotate the saw chain around the guide bar that can pivot during the sawing operation. The guide bar is so located in the sawing box that its pathway of motion is not obstructed by the sawing box. The sawing box is provided with openings that allow sawdust to leave the box.
A dangerous situation that can arise during sawing is breakage of the saw chain. Situations have arisen in which various parts have become loose from the broken chain and been thrown out at high speed as projectiles. A chain breakage in which parts are thrown out is generally known as a “chain shot”. The breakage of the chain is caused principally in that a part of the broken saw chain is thrown backwards and is cut off, cut or split due to the sudden whiplash effect that the chain part exerts against some end surface in openings in the protective parts of the sawing box. The parts that are thrown out can include saw teeth, side links, driving links and other parts. Such parts that fly out into the surroundings can cause dangerous situations, in particular if people or other work machines are located in the vicinity of the chain shot. At a work location with several harvesters that work at the same time it is not always possible to ensure a minimum safety distance.
In order to reduce the problem of chain shots, saw arrangements with various types of chain shot protection have been suggested. One known chain shot protection that is revealed by EP 1,528,852 C has a protective wall that carries out an active pivoting motion together with the guide bar at its rear part. A passive arrangement is known from SE 530,792 C2 that, instead of protecting against parts from a split chain, ensures as far as possible that a broken chain does not break into parts. In order to achieve this, the sawing box is equipped with a screen that has an end surface that has been given a convex curvature with a sufficiently large radius and such a hardness that a part of the broken saw chain is not cut off or split on contact with the end surface, but can slide around the end surface essentially unhindered. Due to the large radius, the arrangement demonstrates a lack of any sharp edges against which the chain can be split. Due to the hard surface, the links of the chain do not cut into the material and are not thus broken off. A timber working head for the cutting of tree trunks is known from SE 517,665. In order to prevent parts from a broken saw chain being thrown out, the sawing box is provided at its rear end edge with a shielding wall that carries out an active pivoting motion together with the guide bar. In one execution, the actively displaceable shielding wall may be manufactured from an elastic flexible material such as rubber. The shielding wall is pivoted by means of a turning motor that works synchronously with the guide bar during its pivoting motion during a cutting operation. The term “passive protection” is used to denote such protection as is mounted stationary in the sawing box.
Each one of the arrangements described above has been shown to function well and has probably resulted in that it has been possible to avoid serious accidents. It should, however, be understood that tree harvesters are advanced units that are subject to heavy effects when working from impacts and collisions, and also from contaminants such as dirt, dust and sawdust that circulate in the air around the timber working head. Mechanisms that are components of the tree harvester can often be affected by disturbances, which lead to unnecessary interruptions in operation. It is, therefore, desirable to minimise as far as is possible the number of mechanisms, and in particular motor-driven mechanisms, that are components of the timber working head, which is in itself already very complex. In particular, with respect to the passive arrangement that is revealed by SE 530,792 C2, the extra saw protection mentioned in the introduction at the actively pivoting saw mechanism is unnecessary.
There is, however, always an aspiration to improve and modify protective arrangements of this type with the aim of achieving both cheaper and more efficient protection.